Setting the TOD clock during IPL

The time-of-day (TOD) clock setting is critical to IMS log integrity and the proper functioning of database recovery. Use caution when setting the TOD clock during system IPL to avoid database integrity and recovery problems.

Attention: Setting the Greenwich mean time (GMT) clock value back at IPL time can cause severe database integrity and recovery problems. Issuing a SET CLOCK command to change the local time, for example at the end of daylight saving time, has no effects on IMS recoverability.

The time-of-day (TOD) clock setting is critical to IMS log integrity and the proper functioning of database recovery, IMS restart, and XRF tracking/takeover. Never set the TOD clock to a time earlier than the immediate prior shutdown or failure without taking actions to reset the recovery base. You can reset the recovery base by invalidating the existing log, image copy, and change accumulation data sets. If the TOD clock must be set to a time earlier than the previous shutdown or failure, you must complete the following procedure to reset the recovery base:

Procedure

  1. Reallocate a different block size for the OLDS data sets.
  2. Reinitialize the DBRC RECON data set.
  3. Make image copies of all database data sets.
  4. Cold start IMS.

Issuing a SET CLOCK command does not reset the TOD clock. You can set the TOD clock only at system IPL either by changing the setting of the sysplex timer (external time reference or ETR); or by replying to the IPL prompts for setting the clock with the GMT option. Therefore, you do not need to reset the recovery base if you issue a SET CLOCK command when the TOD setting must be changed for daylight saving time (for example).